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Matariki amplifier

This project was built as a demonstrator. This amp can be demonstrated either in our Porirua electronics lab, or brought to your location to audition.

This amplifier can be ordered at a cost of $NZ 2200

Its features are:

  • Remote control volume, input selection and standby power
  • Tone controls
  • Phono input
  • Small footprint chassis
  • 15 wpc output

Also, this amplifier was constructed entirely on printed circuit boards, to enable easy re-use of the modules in subsequent projects.

Around the back

Inside

Everything is hand built and assembled (except for the green boards which are outsourced components for the remote control)

Specifications

Main Amplifier

Topology
Line-level amplifier, grounded cathode gain stage, DC-coupled
cathodyne phase inverter, push-pull EL84 output in class AB using fixed-bias ultralinear topology, global negative feedback. 360V Plate Voltage.
Valve complement
Gain stage: 1 X 6N2
Phase Inverter: 1 X 6N1
Output: 4 X EL84
Power Output (measured)
15W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured)
1.5% THD at rated power, 1kHz, resistive load
Output Impedance
4Ohm 8Ohm
Input Impedance
500 KOhms
Input Sensitivity
1v rms for rated power (2.8v p-p)
NFB
10dB
Frequency Response
10Hz – 24kHz ±1dB
6Hz – 32kHz ±3dB
-6dB at 50kHz
Power consumption
230v 50Hz 190w nominal

RIAA Phono Preamp

Sensitivity
4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Gain
+42dB at 1KHz
Freq Response
RIAA Compliant ±1dB (15Hz–20KHz)
Topology
Cascaded dual-triode 6N2 implementing RIAA filter in NFB



Tone Control  

Topology
unity-gain cathode-follower driven baxandall tone stack with NFB
Valve complement
2 X 6N2
Tone adjustment
±12dB at 100Hz and 10 kHz

EL84 Amplifier with extras

This project was commissioned by an overseas customer. The requirements list for this one was complex:

  • Amplifier with remote control volume
  • Tone controls
  • Headphone output
  • Phono input
  • Small chassis suitable for international shipping

For the main amplifier, the same topology and transformers as the previous EL84 amplifier was used. However this one has a higher tube count because of all the extras.

Also, this amplifier was constructed entirely on printed circuit boards, to enable easy re-use of the modules in subsequent projects.

Around the back

Specifications

Main Amplifier

Topology
Line-level amplifier, grounded cathode gain stage, DC-coupled
cathodyne phase inverter, push-pull EL84 output in class AB using fixed-bias ultralinear topology, global negative feedback. 360V Plate Voltage.
Valve complement
Gain stage: 1 X 12AX7 (ECC83)
Phase Inverter: 1 X 12AU7 (ECC82)
Output: 4 X EL84
Power Output (measured)
15W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured)
1% THD at rated power, 1kHz, resistive load
Output Impedance
4Ohm 8Ohm
Input Impedance
500 KOhms
Input Sensitivity
850mV rms for rated power
NFB
10dB
Frequency Response
6Hz – 55kHz ±3dB
Power consumption
230v 50Hz 190w nominal

RIAA Phono Preamp

Sensitivity
4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Gain
+46dB at 1KHz
Freq Response
RIAA Compliant ±1dB
Topology
Cascaded Dual triode ECC83 / 12AX7 implementing RIAA filter in NFB



Tone Control

Topology
unity-gain cathode-follower driven baxandall tone stack with NFB
Valve complement
2 X 12AX7 (ECC83)
Tone adjustment
±12dB at 100Hz and 10 kHz

 

 

Standalone Tone Control

The tone control was a scrapyard project… A few leftover parts from previous projects were sitting in the workshop so the idea was conceived of building something useful to add to the big KT88 amplifier (featured in a previous entry).

Designing the tone control presented the opportunity of designing a tube circuit on a printed circuit board.

By way of explanation: Traditional tube circuits tend to use so-called “point-to-point” wiring, rather than using a circuit board. However, circuit boards allow an item to be designed once and then built multiple times. So there are definite advantages.

In this case, an existing published circuit schematic was used, with some changes to make it more balanced. A power supply was designed – again using existing parts to hand, and a bypass capability was added into the design of the board.

The idea of placing all these electronics within view led to the replacement of the aluminium case top with clear acrylic, to allow easy viewing of the insides.

EL84 amp with multiple speaker outputs

This was a project for a customer with a unique requirement: The amplifier needed to have the option to connect two sets of speakers. This customer had a “man cave” containing a formidable collection of speakers acquired from auctions and secondhand shops. So the need was to connect more than one pair and be able to switch between them.

Also required was a phono stage, since the customer had a large collection of vinyl and a wonderful vintage Audio-Research turntable from the late ’60s.

The amp was built on a small chassis to fit into the available space. The switch on the front is the speaker selector, the power switch is at the back.

This amplifier is unusual in that it does not have remote control. Usually we supply amplifiers with at least a remote control for the volume, or sometimes for the input selector as well. However this customer was quite adamant it wasn’t needed.

Specifications

Main Amplifier

Topology
Line-level amplifier, grounded cathode gain stage, DC-coupled
cathodyne phase inverter, push-pull EL84 output in class AB using fixed-bias ultralinear topology, 10dB global negative feedback. 360V Plate Voltage.
Valve complement
Gain stage: 1 X 12AX7 (ECC83)
Phase Inverter: 1 X 12AU7 (ECC82)
Output: 4 X EL84
Power Output (measured)
15W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured)
1% THD at rated power, 1kHz, resistive load
Output Impedance
4Ohm 8Ohm
Input Impedance
50 KOhms
Input Sensitivity
800mV rms for ratedpower
Frequency Response
6Hz – 55kHz ±3dB
Power consumption
230v 50Hz 190w
nominal

RIAA Phono Preamp

Topology
Phono-level amplifier, cascaded grounded cathode gain stage,
DC-coupled cathode follower, RIAA equalisation
Sensitivity
4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Valve complement
Gain stage: 2 X 12AX7 (ECC83)
Cathode Follower: 1 X 12AU7 (ECC82)

 

Listening tests

The sound from this one is clean, detailed and very pleasing. The main amp stage is based on the well-regarded Fisher X100, with the 12AX7 gain and 12AU7 phase splitter, though this design runs that 12AU7 closer to its 5mA sweet spot for linearity from a 300V B+ than the Fisher does.

The output stage is EL84 in fixed bias ultralinear, biased to 8.4W quiescent dissipation (70% of rated maximum)

The RIAA stage works well. The Cathode follower is needed to drive the volume control which represents a 50K load across the input. The noise floor is low, hum is non-existent, and distortion does not occur even on the loudest passages.

KT88 amp – the first build

This amp was the grand-daddy – the first build for ATRAD-Audio. It remains under our ownership as a daily driver. The aesthetics of this one are such that only a mother could love it, but the point was to build something as a learning exercise.

Everything about this amplifier was designed and built from the ground up.

Some specifications

  • Topology: Williamson
  • NFB: 14dB
  • Valve complement (per channel): 1 x 12AX7. 1 X 12AU7, 2 X KT88
  • B+ Voltage: 560V
  • Bias: 28W disspiation (50mA)
  • Measured power output: 80W into 4Ω at 1% THD (1kHz)
  • Freq. response: -3dB points at 13Hz and 46kHz
  • Weight: (approx): 27kg
  • Mains transformer: Custom-made
  • Output transformers: Hammond 1650R

 

Around the back

This unit was built with some concessions to convenience. A through-power port, to allow connected accessories to switch on with the main amplifier, in this case a separate phono pre-amp, and a USB power port with  a separate regulated supply, to power a Chromecast Audio.

 

How does it sound?

Glad you asked! Right from the initial listening tests, we were amazed at the clean, precise and immensely powerful sound from this machine. It replaced a Sony integrated amp of the same power rating, and everything about this one just sounded better. In the bass register the extension, power and control made the Sony look like a weakling. The clarity, warmth and imaging of the sound invites long listening sessions without fatigue.

For potential customers, wanting to get a flavour of the sound available from these all-valve amplifiers, this is a hard one to beat. Just don’t look too hard at it.

 

About the name

Waru Titiwai – it means “eight glow worms”

Eight because there are 8 valves on it, Glow-worms because the valves glow.